1. Field of the Invention
The present invention relates to a servo motor control system that has, in a controller, a serial bus control circuit having a function of sending a current command in a plurality of different control modes to servo amplifiers for driving a servo motor.
2. Description of the Related Art
Driving and controlling a large-capacity motor requires a large-capacity inverter unit. However, acquisition of a large-capacity inverter unit is difficult because of constraints introduced by its components. Accordingly, a plurality of small-capacity inverter units are connected in parallel to obtain a large capacity required to drive and control a large-capacity motor.
When a motor control unit supplies a motor driving command to each of the plurality of small-capacity inverter units connected to drive the motor, the motor control unit needs to generate as many motor driving commands as the inverter units. In this case, a plurality of motor driving command generation units are exclusively used to drive a motor. As a result, when the motor control unit drives another motor in addition to the above one, a certain number of motor driving command generation units for driving the other motor are further required, thereby limiting the total number of motors that can be driven.
A motor control system that controls a plurality of inverter units using one motor driving command from a motor control unit and thereby drives one large-capacity motor is disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-86918. This motor control system will be described with reference to FIGS. 7A and 7B.
A motor driving command, which is created by a motor control unit 20 and sent through a serial bus 24 from the motor control unit 20, is supplied through a relay unit 21 to a plurality of inverter units 22a to 22d in parallel. Each of the plurality of inverter units 22a to 22d is controlled by the same (common) motor driving command and drives a motor 23. As described above, the motor control unit 20 can control the plurality of inverter units 22a to 22d using only one motor driving command generation unit. Accordingly, when the motor control unit 20 drives one or more other motors in addition to the motor 23 above, it is enough to add one or more motor driving command generation units and the addition is not so difficult.
The relay unit 21 in FIG. 7A has a receiver circuit 21a, a setting circuit 21b, a computation circuit 21c, and a plurality of driver circuits 21d1 to 21dn as shown in detail in FIG. 7B. The plurality of driver circuits 21d1 to 21dn correspond to the plurality of inverter units 22a to 22d, respectively. The motor driving command (torque command) output as parallel data from the motor control unit 20 is received by the receiver circuit 21a of the relay unit 21 and input to the computation circuit 21c. The computation circuit 21c generates the motor driving command for the plurality of inverter units 22a to 22d. The generated motor driving command is simultaneously output as parallel data from the computation circuit 21c to the plurality of inverter units 22a to 22d through the plurality of driver circuits 21d1 to 21dn. 
The motor control system in FIG. 7A has the relay unit 21 (see FIG. 7B) on the serial bus 24 that connects the motor control unit 20 and the motor 23. Therefore, a unit for controlling the motor 23 becomes complicated, making downsizing difficult. In addition, a command signal for the motor 23 is received by the relay unit 21 as serial data and, based on the received data, the command signal for the motor 23 is created, so response delay is caused.